This invention concerns the utilization of the heating values of carbonaceous fuels for the production of useful thermal, mechanical or electrical energy.
Burning coal to generate steam is one of the oldest of the industrial arts. Numerous inventions have been applied to improving its efficiency and alleviating the coproduction of noxious smoke, which tends to contain unburned fuel, finely powdered ash and oxides of sulfur and nitrogen. Nevertheless, even with the latest technology, coal is considered a dirty fuel, capable only with great difficulty and expense of complying with increasingly stringent air pollution standards.
The high cost of removing sulfur oxides from conventional flue gases has resulted in a spread between the prices of low and high sulfur coals. Moreover, the former are found, for the most part, in western states remote from the areas of greatest energy need. Thus the market price structure provides economic incentive for the commercialization of a process able to produce steam and power from high sulfur coals without air pollution. Other reserves of solid fuels remain largely untapped because of high contents of water or ash.
Combustion of coal in conventional ways creates temperatures well over 2000.degree. F. Conventional apparatus must, therefore, be constructed of expensive materials capable of withstanding such temperatures. Moreover, components of the ash frequently melt or sinter forming deposits which foul parts of the apparatus, causing loss of efficiency, downtime and increased maintenance expense. A further undesirable consequence of the usual temperatures is the inadvertant formation of nitrogen oxides which cannot be effectively and economically removed from flue gas with available technology.
Generation of high pressure steam does not inherently require such high temperatures since the boiling point of water at 2000 pounds per square inch is only about 635.degree. F. and at 3000 pounds per square inch under 700.degree. F.
It has been proposed to burn coal by the indirect means of first converting it to liquid or gaseous fuel, which can be desulfurized before combustion to a clean flue gas. These techniques also employ high temperatures and generally share serious economic and operational drawbacks associated with coal's tendency to cake and stick when heated, the formation of soot or tarry residues and difficulties with erosion and dust control. They are further burdened with low overall thermal efficiencies.
The catalytic effect of common alkalis such as soda ash (sodium carbonate) and limestone (calcium carbonate) on the reactivity of carbonaceous materials is well known and has been utilized in the gasification of coal and coke. Alkaline compounds are used in commercial steam-hydrocarbon reforming catalysts to promote the oxidation of carbon to gaseous products. Conventional combustions do not employ alkaline catalysts because, at the high temperatures, they would volatilize and/or combine with ash ingredients to form troublesome slag or clinker.
Some of the newer fluidized bed combustion processes do, however, use beds containing limestone, or similar alkaline particles, and thus are able to burn the fuel at reduced temperature, avoiding or minimizing nitrogen oxides and slag or clinker. Generally, two categories of fluidized bed combustion processes are recognized in the art: atmospheric pressure and pressurized. While both are considered to have commercial promise, the atmospheric version requires a high excess of alkali to effect even moderately high sulfur removal and both encounter difficulty in separating dust from flue gas. Although pressurized fluidized beds achieve a better alkali utilization, sulfur removal is still incomplete and dust control is even more cruical since energy must be recovered from hot flue gases by expanding them through turbines subject to erosion.
It has been known for more than 70 years that water accelerates the reaction between coal and atmospheric oxygen. Ordinary combustion processes cannot take advantage of this phenomenon because wet fuel must be dried before it will ignite. Moreover, water entering a conventional combustion, as well as the known fluidized bed combustions, leaves the system as vapor, carrying with it as an energy loss its latent heat of evaporation.
The combustion-promoting effect of water is strikingly illustrated by a family of processes, known as Wet Air Oxidation (WAO), which modify or destroy organic matter suspended in water by contact with air at elevated temperature and pressure. While used mainly to purify waste water WAO, which was originally known as the Zimmerman process, has been proposed as a means of desulfurizing coal by partial oxidation and even for recovering energy from such fuels as peat. WAO is liquid phase and therefore confined to temperatures below the critical temperature of water (705.4.degree. F.), which limits reaction rates (requires large, expensive reactors) and the temperature at which useful heat can be delivered. WAO processes do not use alkaline catalysts.
There is a large body of art concerning the physical behavior of fluidized solids in general, as functions of particle size and size range, electrical properties, gas velocity, reactor size, proportions, etc. as well as considerable art concerning various versions of fluidized bed combustion. Much of this art is useful background for the behavior of fluidized beds in the process of my invention when proper allowance is made for the unusually high gas densities and low gas volumes.
It is customary to express the gas velocity in a fluidized bed as though there were no solids present, the so-called "superficial velocity". The most common form of fluidized bed, in which this velocity is usually between 1 and 10 feet per second (fps), is also described as a "dense phase" to distinguish it from a "dilute phase" of gas, carrying a comparatively light loading of solid particles, usually encountered in a settling space, or freeboard, above the dense phase. At relatively low gas velocities and/or with coarse and/or dense particles, there may be a clearly defined interface between dense and dilute phases, rather resembling a liquid level. On the other hand, with high gas velocities and/or fine and/or light particles, the transition may take place over a zone of appreciable depth in which there is a gradient between dense and dilute phase particle loadings.
Following the descriptions of the embodiments will be easier if it is understood that the art of fluidized solids processes generally recognizes two additional gas-solids phase conditions, viz., transport or "entrained phase" and compacted or "settled phase". Compared with dense phase, entrained phase is characterized by high gas velocity and low particle loading. It is found in gas-solids inlet lines and risers or "draft tubes". At the opposite end of the velocity-density spectrum is settled phase, found in drawoff wells, standpipes and hoppers, in which gas velocity is comparatively low and particle loading comparatively high. A high degree of turbulence and mixing is found in an entrained phase, a moderately high degree in a dense phase and a low degree in a settled phase. These phase distinctions are a convenience to the technical writer but there can be intermediate conditions that are not clearly one or the other. For example, characteristics of a so-called dilute phase can approach those of an entrained phase.
Flue gas is nearly always a combustion by-product of zero, or even negative value (considering its unrecovered heat and the expense of meeting emissions regulations). However, in recent years an application of pressurized combustion has emerged, in which both flue gas and its pressure energy are valuable products. This application is the enhanced recovery of residual oil from old oilfields.
Carbon dioxide in flue gas under pressure tends to dissolve in crude oils, increasing their fluidity. The pressure and volume of the undissolved gas helps to move the oil toward a producing well. Fluidity can also be increased by heating so, in some cases, steam is injected into the formations to raise their temperature. Later technology combines the benefits of both by injecting flue gas-steam mixtures, generated by combustion of a liquid or gaseous fuel under pressure, followed by quenching of the hot combustion products with water.
At the high temperature of these essentially conventional combustions, nitrogen oxides are formed. In the case of sulfur-containing fuels, sulfur dioxide is produced. These pollutants are removed, to a degree, by adding chemicals to the quench water. Disadvantages of the present art are the high cost of premium fuels, inadequate pollutant control, costs of chemicals and of disposing spent chemicals. There is substantial economic incentive to substitute cheaper solid fuels and to reduce treatment and disposal expense.